Ironically, given what I just said, it's likely because of sex education at too early an age. Introducing pre-teens to the concept of sex (never mind peculiarities like gay sex), in such a structured and authoritative environment, is going to give them very odd ideas anyhow it works. In times past, kids would learn about sex organically, either because they lived in a single-room hovel, or because they worked on a farm.
Having someone awkwardly wrap a dildo (or banana) in a condom, then start waxing lyrical about fisting, would fuck anyone up. Kids in particular are going to hyperfocus on the topic, because that's just how children work, especially when they're low-key traumatised.